This work continues and further develops a highly successful collaboration among several institutions in the US and Brazil to investigate the interactions of low-energy electrons with polyatomic molecules, and in particular with biofuels and other biological molecules. The collaboration includes scientists from California State University, Fullerton, the California Institute of Technology, the State University of Campinas. It involves both experimental and theoretical groups who have come together to work jointly on problems where they have overlapping interests and complementary expertise. Major focuses in future work will be electron-impact excitation of polyatomic molecules and dissociative electron attachment to polyatomics. Calculations and measurements of electron-impact excitation processes in polyatomics are challenging, and results are scarce despite the need for such data in understanding reactive plasmas and discharges. To build expertise in this area, measurements and calculations are carried out for two prototypical molecules, ethylene and furan, both of which have low-lying triplet states well isolated from other thresholds. Following that, measurements and calculations of electron-impact excitation cross sections for methanol and ethanol will be done. Electron-impact excitation of these prototypical and ubiquitous alcohols is of fundamental interest and also has high technological relevance: dissociative excitation and ionization are key processes in spark ignition of alcohol fuels, and rate data are needed for modeling and optimizing the ignition process. Collaborators in Brazil affiliated with the Bioethanol Science and Technology Center in Campinas are initiating studies of spark ignition that will involve both laboratory measurements and numerical modeling, and those studies will make direct use of the electron-impact excitation cross sections that will be measured and calculated as part of this project.
The broader impacts are that electron-molecule collisions at low energy present many features of fundamental interest and pose a great challenge to both experimental and computational methods, but they also have wide technological significance, being relevant to the physics of plasmas and discharges, the upper atmosphere, and circumstellar and interstellar media. In the specific case of methanol and ethanol, accurate cross-section data for the major collision processes, including elastic scattering, excitation, and ionization, are needed for numerical modeling and optimization of discharges used to ignite alcohols used as fuels. Such modeling may lead to improved spark-plug designs that produce more complete combustion and thus higher fuel efficiency. Moreover, an understanding of electron driven excitation and dissociation in the alcohols may give insights into the same processes in related biomolecules, including sugars. A central component of the project is providing opportunities for young scientists to gain experience doing science in an international context, with US students making extended visits to do research in Brazil and vice versa.
This was a collaborative project involving our theoretical research group at the California Institute of Technology, the experimental group of Professor Murtadha Khakoo at California State University, Fullerton, and several experimental and theoretical research groups in Brazil. Together, we set out to study the interaction between low-energy electrons and a number of small molecules. The project had both fundamental and applied aspects. From the point of view of basic science, we wanted to understand similarities and differences in how related molecules scatter electrons, and we wanted to improve both experimental and computational procedures for studying inelastic collisions, that is, those that leave the molecule in an excited quantum state. At the same time, we aimed to provide the sorts of data needed when modeling electron transport and electron-driven chemistry. One area we focused on was alcohols. Ethanol is heavily used as a renewable replacement for gasoline in the US and even more so in Brazil, and our Brazilian colleagues were interested in data that could assist in optimizing spark ignition in alcohol-powered engines, thus improving performance and efficiency. In previous work, we had studied elastic electron scattering by ethanol, methanol, propanol, and butanol; in the present project, we followed up that work by looking at isopropanol. We found that, as we had seen in other alcohols, isopropanol's scattering pattern appears to reflect its structure. That is, at collision energies where the electron interacts closely with the molecule, the angular distribution of electrons was characteristic of other branched alcohols, whereas straight-chain isomers show a different pattern. We also took first steps toward studying inelastic collisions that can dissociate alcohols and initiate chemistry by completing extensive laboratory and computational studies of electron-impact excitation of water. As the most abundant biological molecule, water is of great importance in its own right, but it is also a model for alcohols; its chemical formula is HOH, while that of alcohols is ROH, with R representing an alkyl group. Our measured and calculated results agreed fairly well but differed significantly from the only previous measured data. We believe many of those differences can be explained by defects in the data analysis applied to that earlier experiment. Following up on our study of water, we began a study of electron-impact excitation of methanol, work that is continuing both at Cal State Fullerton and at Caltech. Another area we are interested in is electron-driven chemistry in biological systems. Here we want to understand both elastic and inelastic electron collision processes involving biological molecules. We especially want to understand dissociative attachment, in which the electron becomes permanently attached as the molecule breaks up into two or more fragments, because even quite slow electrons can initiate this reaction. As part of this project, we studied elastic scattering by the prototypical alkyne molecule, acetylene (C2H2), and by tetrahydrofuran, a ring-shaped molecule that is a model for the deoxyribose component found in the backbone of DNA (deoxyribonucleic acid), the polymer that carries genetic information in living cells. As a step toward studying dissociative attachment in biological molecules, we initiated studies of a simpler system, methyl chloride (CH3Cl); this work is going forward at the University of Sao Paulo in Brazil.
